Everything about Electron Transfer totally explained
Electron transfer (ET) is the process by which an
electron moves from one
atom or
molecule to another atom or molecule. ET is a mechanistic description of the thermodynamic concept of redox, wherein the formal oxidation states of both reaction partners change.
Numerous processes in biology involve ET reactions, including oxygen binding, photosynthesis, and respiration. Many metabolic pathways as well as many detoxification routes also involve ET. Additionally, the process of
energy transfer can be formalized as a two electron exchange (two concurrent ET events in opposite directions). ET reactions commonly involve transition metal complexes, but there are now many examples of ET in organic molecules.
Classes of electron transfer
There are several classes of electron transfer, defined by the state of the two redox centers and their connectivity
Inner-sphere electron transfer
In inner-sphere ET, the two redox centers are covalently linked during the ET. This bridge can be permanent, in which case the electron transfer event is termed intramolecular electron transfer. More commonly, however, the covalent linkage is transitory, forming just prior to the ET and then disconnecting following the ET event. In such cases, the electron transfer is termed intermolecular electron transfer. A famous example of an inner sphere ET process that proceeds via a transitory bridged intermediate is the reduction of [CoCl(NH
3)
5]
2+ by [Cr(H
2O)
6]
2+, as described by Taube. In this case the chloride
ligand is the bridging ligand that covalently connects the redox partners.
Outer-sphere electron transfer
In outer-sphere ET reactions, the participating redox centers are not linked via any bridge during the ET event. Instead, the electron "hops" through space from the reducing center to the acceptor. Outer-sphere ET is by definition intermolecular. Outer sphere electron transfer can occur between differing chemical species or between identical chemical species that differ only in their oxidation state. The later process is termed self-exchange. As an example, self-exchange describes the
degenerate reaction between
Permanganate and its one-electron reduced relative
manganate:
» [MnO
4]
- + [Mn*O
4]
2- → [MnO
4]
2- + [Mn*O
4]
-
A key concept of
Marcus theory is that the rates of such self-exchange reactions are mathematically related to the rates of "cross reactions". Cross reactions entail partners that differ by more than their oxidation states. One example (of many thousands) is the reduction of permanganate by
iodide to form
iodine and, again, manganate.
Heterogeneous electron transfer
In heterogeneous electron transfer an electron is moved between a chemical species and a solid-state
electrode. Theories addressing heterogeneous electron transfer have applications in
electrochemistry and the design of
solar cells.
Theory
The first generally accepted theory of ET was developed by
Rudolph A. Marcus to address
outer-sphere electron transfer and was based on a
transition-state theory approach. The Marcus theory of electron transfer was then extended to include
inner-sphere electron transfer by
Noel Hush and Marcus. The resultant theory, called
Marcus-Hush theory, has guided most discussions of electron transfer ever since. Both theories are, however,
semiclassical in nature, although they've been extended to fully
quantum mechanical treatments by
Joshua Jortner,
Alexender M. Kuznetsov, and others proceeding from the
Fermi's Golden Rule and following earlier work in
non-radiative transitions. Furthermore, theories have been forwarded to take into account the effects of
vibronic coupling on electron transfer. In particular the
PKS theory of electron transfer.
Further Information
Get more info on 'Electron Transfer'.
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